The present invention relates to a process for reducing concentrations of hair, trash, or fibrous materials, in a waste water treatment system, particularly a membrane bioreactor.
Minimizing hair, trash, or fibrous materials build-up on membrane filters in waste water treatment systems is desirable for efficient operation of the system. Hair, trash, or fibrous materials, can accumulate on the membrane fibre bundles of the membrane filter and are difficult to remove and can damage the membrane.
Conventional processes to reduce hair, trash, or fibrous materials build-up can include screening the raw sewage stream to the waste water system before the raw sewage enters the waste water treatment system. Another approach screens the entire sludge stream (comprised of raw sewage plus sludge recycle from the system) before it enters the waste water treatment system.
Screening the raw sewage, however, requires that the screening system be designed to accommodate the raw sewage flow under peak conditions which can typically be three or more times the average design flow rate of the system. This requires that the screens be large to accommodate the flow. Additionally, such system should be designed with duty screens and standby screens each of 100% of the design screen capacity. By screening the raw sewage before it enters the waste water treatment system, the screenings generated can be high in undigested organic materials that require special equipment to return the undigested organic materials from the screenings to the waste water treatment system. Moreover, screening raw sewage before it enters the waste water treatment system does not remove hair, trash, or fibrous materials that can form within the system itself, such as, for example, re-roped or re-balled fibrous materials.
Screening the entire sludge stream before it enters the membrane filter of the waste water treatment system must also accommodate high flow rates (typically four or more times the average design flow rate of the system) again requiring the use of large duty screens and full capacity standby screens.
The present invention relates to a process for reducing hair, trash, or fibrous material concentration in a waste water treatment system. Waste water treatment systems of this invention have a filter, such as, for example, a membrane filter or a hollow fibre membrane filter, in conjunction with a bioreactor to produce a treated effluent from the filter and waste sludge from material rejected by the filter or otherwise remaining in the bioreactor. The bioreactor can be located upstream of the filter in the waste water treatment system or alternatively the filter can be in the bioreactor or a part of the bioreactor. In the latter instances the membrane of the filter is submerged directly in the bioreactor.
The process comprises flowing water to be treated to the waste water treatment system. A portion of mixed liquor from the waste water treatment system is removed and passed through a screen to remove hair, trash, or fibrous materials from the mixed liquor. The screened mixed liquor is then flowed to the waste water treatment system. The flow rate of the mixed liquor through the screen is not more than about the average design flow rate of the waste water treatment system. The flow rate of the mixed liquor through the screen can be about 0.10 to about 1.0 of the average design flow rate of the waste water treatment system. In particular, the flow rate of the mixed liquor through the screen can be not more than about half the average design flow rate of the waste water treatment system. Moreover, the mixed liquor can pass through the screen at a substantially constant flow rate and can pass through the screen substantially continuously.
The mixed liquor to be screened can be removed from the recycle mixed liquor stream in the waste water treatment system. Typically recycled mixed liquor is recycled to the head of the waste water treatment system, and the mixed liquor to be passed through the screen can be removed from the recycle mixed liquor stream. For example, the mixed liquor to be passed through the screen can be removed from the recycle mixed liquor stream upstream of where the screened mixed liquor flows back to the recycle mixed liquor stream. Alternately, the mixed liquor to be screened can be removed from the waste water treatment system at various other locations. The screened mixed liquor can be flowed back to the wastewater treatment system directly or to the recycle mixed liquor stream and the screenings can be either treated or disposed of directly or in combination with the waste activated sludge.
The openings of the screen can be not more than about 1.0 mm. Particularly, the openings can be not more than about 0.75 mm. More particularly, the openings can be not more than about 0.50 mm. One screen suitable for use in this invention is, for example, a rotary drum screen. The invention can be used, without limitation, with waste water treatment systems with existing screens or clarifiers on raw sewage that do not meet minimum screening requirements for membrane filters, with systems meeting minimum screening requirements where the operator desires added protection for a membrane filter, with new systems where the invention can be used in place of any other screening and in new or existing plants that will use the invention to provide both screening and sludge thickening.